Two-lens telescope objective with reduced secondary spectrum



`."L.f.`\\.,v 1"\V"\j,4 OR 2 s 8 5 0 o 945 T 9H La :2,

Sept. 9, 1958 H. KHLER 2,850,945

Two-LENS TELEscoPE OBJECTIVE WITH REDUCED SECONDARY SPECTRUM Filed June18, 1956 )Q 2 o S' E United States Patent TWO-LENS TELESCOPE OBJECTIVEWITH REDUCED SECONDARY SPECTRUM Horst Khler, Heidenheim (Brenz),Wnrttemberg, Germany, assignor to Carl Zeiss, Heidenheim (Brenz),Wnrttemberg, Germany Application June 18, 1956, Serial No. 592,216

2 Claims. (Cl. 88-57) The present invention concerns an improvement intwo-lens telescope objectives with the aim to reduce the secondaryspectrum in telescopes of this kind. The invention refers to objectivesof the kind that is comprised of one collective lens and of onedispersive lens separated from it by an air-space.

The theory of telescope objectives which consist of two thin lenses,states that the secondary spectrum is given by the relation:

on the assumption that the primary chromatic aberration is corrected forthe colours C and F. As is then in the relation stated above thelongitudinal aberration for a third colour for which the relativepartial dispersion referring to the first lens is tpl, and to the secondlens is tpg. Furthermore f designates the focal length and v the Abbenumber for the glasses of these lenses.

Generally one considers the secondary spectrum for the violet mercuryline g; then 7l n@ npnc In order to reduce the secondary spectrum onemust select a pair of glasses for which the expression is as small aspossible. In the two-lens objectives so far known normally a pair ofglasses was used whose collective lens consisted of an ordinary crownglass and whose dispersive lens consisted of a so-called telescope flintglass. Now telescope ints do guarantee suitable values of v and of tp sothat the factor Even with heavy flint glasses in the rear dispersivelens the v-dilerence in the denominator of the equation is relativelyhigh which is concurrently decisive for diminuting the numerical valueof said equation. With heavy ints for the dispersive lens in combinationwith crown glass for the collective lens however, the difference of therelative partial dispersion in the numerator of the equation will berelatively high too. This frustrates the lia 2,850,945 Patented Sept. 9,1958 success aimed at with regard to considerably reducing the secondaryspectrum of two-lens telescope objectives. The present invention takesadvantage of a not generally known property of lanthanum ints accordingto which with their high refractive ndices their numerical values of theAbbe number v and of the relative partial dispersion tp are similarlyplaced to those of the so-called telescope ints. The invention consistsin combining a lanthanum int glass of this type with such a crown glassthat a suiciently large difference of the r-values i.e. for :f1-v2occurs, so that in order to comply with the conditions for achromatismthe refractive surfaces can be given lower values of curvature, and thatmoreover these glasses have a smaller difference of their relativepartial dispersions (tpl-tpg). Finally it is a special advantage for theinvention to forgo cementing of the two lenses and to introduce anair-space between the two lenses. Following these recognitions theobject of the present invention is a two-lens telescope objective withreduced secondary spectrum combined of a collective lens and of adispersive lens separated from it by an air-space wherein according tothe invention the dispersive lens consists of a glass of the group oflanthanum ilints whose Abbe number is greater than 35 but smaller than52 and whose relative partial dispersion JLA- W np-ng is greater than1.56 and whose refractive index for the yellow helium line is greaterthan 1.7, and the collective lens consists of a glass whose Abbe numberr is greater than 65, and that furthermore the air-space amounts to amaximum of 5% ofthe focal length.

In the attached illustration an objective according to the invention hasbeen represented for which numerical values are listed in the tablegiven in the following. In the gure and in the table there aredesignated By L, the lenses.

By r, the radii of the surfaces.

By d, the separations of the surfaces from each other. By nd, therefractive indices.

By v, the Abbe numbers.

The data given in the table refer to a focal length of f- 1.000.

Lens I Lens II nc= 1. 4853 1. 7398 m l. 4876 l. 7448 m= 1. 4923 1. 7564n.= 1. 4960 1. 7658 v., 7o. 0 44. 7 w: 1. 527 1. 563

d1=0. 0878 dz=0. 00928 d;=0. 0299 x=0. 91484 r1=+0. 49155 n=-0. 43806 n=-0. 43177 n=1.23460 element, the focal length of said front collectivelens element and the focal length of said rear dispersive lens elementbeing so related that the two said lens elements are axially separatedapart by an air space amounting at least to .5% and at most to 5% of thefocal length of the objective, the said front collective lens elementconsisting of a glass whose Abbe number is greater than 65, the saidrear dispersive lens element consisting of a glass whose refractiveindex for the yellow helium linc nd is greater than 1.70 and whose Abbenumber is greater than 35 but smaller than 52 and whose relative partialdispersion "np-ng is greater than 1.56, in said front collective lenselement the radius of the front surface having a value between .35 f and.6U-f and the radius of its rear surface having an absolute valuebetween .35 f and .55 f, in said rear dispersive lens element the radiusof the surface directed towards said front collective lens elementhaving an absolute value between .35'f and .S5-f, f being the focallength of the whole objective.

2. A two-lens objective lens system according to claim 1 with theconstructive data of radii, apical separations and glasses correspondingto the values to be taken from the following table to an extent that thepower of refraction (An/r) of each surface lies within a range ofi-.51/f and each apical separation lies within a range of :.2- f on bothsides of the said values:

4 L being the lenses; r being the radii; d being the apical separations;nd being the refractive indices; vd being the Abbe numbers; An/r beingthe power of refraction of the surfaces, and f being the focal length ofthe objective lens system.

References Cited in the le of this patent UNITED STATES PATENTS 576,896Rudolph Feb. 9, 1897 2,453,218 Grey Nov. 9, 1948 2,764,064 Kohler Sept.25, 1956 FOREIGN PATENTS 104,343 Germany July 28, 1899 189,255 GermanySept. 30, 1907 847,600 France July 3,' 1939 116,659 Australia Mar. 2,1943-

